University of Kentucky College of Agriculture Plant Pathology Extension CooPeRative exTension SeRvice UnIversity oF Kentucky College oF agriculture, FooD anD environmenT Plant Pathology Fact Sheet PPFS-FR-T-04 Black Knot Nicole Ward Gauthier Dennis Morgeson Extension Plant Pathologist County Extension Agent Introduction Cause & Disease Development Black knot is a common, often serious, disease of Black knot is caused by the fungus Apiosporina plums and cherries in Kentucky. Ornamental Prunus morbosa (syn. Dibotryon morbosum), which species, as well as wild plums and cherries, may also overwinters in knots on previously infected twigs be affected. Trees in both commercial and residential and branches. Spores develop within knots in plantings are susceptible. spring between bud break and shuck split and are spread by wind and rain. Only elongating (actively Symptoms growing) twigs of the current season’s growth are susceptible. While infection takes place in spring, Black knot is aptly named for the conspicuous black knot development is not evident until autumn. knotty growths that form on infected branches (Figure 1). Initially, however, these irregular swellings or knots are small and light brown (Figure 2). One year after infection, the enlarging knots become olive-green with a velvety surface. As the season progresses, swellings harden, become brittle, and turn black in color (Figure 3), reaching lengths of 6 inches by the end of the growing season. Often only one side of a limb is affected (Figure 4); but, in some cases, limbs may become completely encircled. Knots continue to expand each year until girdled branches eventually die. FIGURE 1. BLACK KNOT DISEASE IS EASILY RECOGNIZED BY THE BLACK KNOTTY GROWTHS ON TWIGS AND BRANCHES. agriculture & natural Resources • Family & Consumer Sciences • 4-H/youth Development • Community & economic Development FIGURE 2 FIGURE 3 FIGURE 4 Sanitation All diseased wood should be discarded and/or destroyed; avoid leaving infective clippings on the orchard floor. Removal of wild plum and cherry trees in the vicinity of cultivated trees can help reduce inoculum (infective spores). Fungicides If black knot is a persistent problem, fungicide sprays are recommended in spring to protect young, expanding twigs. Fungicides are not curative, and they are ineffective unless an eradicative program of pruning and sanitation is followed. For specific FIGURE 2. EARLY INFECTIONS APPEAR AS LIGHT BROWN SWELLINGS. recommended fungicides, homeowners should refer FIGURE 3. KNOTS BECOME BLACK AND BRITTLE AS THE GROWING to Extension publication, Disease and Insect Control SEASON PROGRESSES. FIGURE 4. BLACK KNOT CANKERS OFTEN DEVELOP ON ONE SIDE OF A BRANCH, EXPANDING EACH YEAR. for Home Grown Fruit in Kentucky (ID-21), while commercial growers should refer to Midwest Tree Disease management Fruit Spray Guide (ID-92). Pruning and sanitation are the primary means for reducing or eliminating black knot. Fungicide additional Resources applications may be used in conjunction with cultural • Commercial Tree Fruit Spray Guide (ID-92) practices, but are often not warranted. http://www2.ca.uky.edu/agcollege/plantpathology/ ext_files/PPFShtml/MwTreeFruitSprayGuideID92. Pruning pdf Pruning is best accomplished in autumn or winter • Disease and Insect Control Program for after leaves fall. This makes it easier to locate Homegrown Fruit in Kentucky, including Organic branches with galls. Cuts should be made at the Alternatives (ID-21) point of the branch union, at least 6 to 8 inches http://www.ca.uky.edu/agc/pubs/id/id21/id21.pdf below knots. A second inspection in April (before buds open) is recommended to remove any newly acknowledgement formed knots. Complete removal of trees may be The authors thank Paul Vincelli, UK Extension Plant necessary if trunks and large limbs become girdled. Pathologist, for his review of this publication. December 2015 Photos: University of Kentucky—Dennis Morgeson (figure 1), John Strang (figure 2) & Nicole Ward Gauthier (figure 4); Bugwood.org—Bruce Watt, University of Maine (figure 1 inset) & Ansel Oommen (figure 3) Educational programs of the Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability, or national origin. .